The present invention relates to an open flow cryostat for cooling a sample in use.
Open flow cryostats are provided for directing a flow of a cryogen, such as helium, over a sample causing the sample to be cooled. This is typically used for cooling crystals to allow the crystal to be examined using X-ray diffraction, neutron diffraction, or other similar techniques.
However, such apparatus suffers from the drawback that large quantities of cryogen must be vented into the atmosphere in order to cool the sample. This coupled with a loss in efficiency caused by warming of the cryogen during transport from a supply vessel to the sample means that open flow cryostats tend to require large volumes of cryogen in order to operate.
In addition to this, problems can occur with ice formation on the sample crystal. A method of avoiding this problem is proposed in U.S. Pat. No. 6,003,321. This document describes a cryostat system which provides a primary helium flow over a sample crystal to cause the crystal to be cooled. In addition to this, a secondary helium flow is provided radially outwardly from the primary helium flow at a slightly warmer temperature. The secondary helium flow tends to help prevent the formation of ice on the sample crystal.
However, in this particular technique, this further increases the amount of helium required to operate the cryostat, thus making operation of this form of open flow cryostat extremely expensive.
In accordance with a first aspect of the present invention, we provide an open flow cryostat for cooling a sample in use, the cryostat comprising:
a. A supply for supplying a coolant;
b. An outlet for directing a flow of the coolant towards the sample;
c. A supply line for transporting coolant from the supply to the outlet; and,
d. An isolation line arranged to transport at least some of the coolant away from the outlet, the isolation line being positioned in contact with at least a portion of the supply line to thermally isolate the supply line from the surroundings.
Accordingly, the present invention provides an open flow cryostat for cooling a sample. The cryostat includes a supply line for transporting coolant from a supply to an outlet, and an isolation line arranged to transport at least some of the coolant away from the outlet. The isolation line is positioned in contact with a portion of the supply line so that the redirected coolant flowing in the isolation line will act to thermally isolate the supply line from the surrounding environment. This helps reduce the heating of the coolant within the supply line which is caused by the higher temperature of the surroundings, thereby improving the efficiency of the cryostat.
The isolation line is preferably arranged coaxially with and radially outwardly from the supply line. This ensures that the entirety of the supply line is thermally isolated from the surroundings. However, other configurations, such as spiraling the isolation line around the supply line could also be used.
A dewar is optionally positioned between the supply line and the isolation line for at least some of the supply line length. This helps provide further thermal isolation of the supply line from the surrounding environment, thereby reducing the heating effect of the surroundings on the coolant as it is transferred to the outlet.
Typically the cryostat further comprises a second supply for supplying a shielding coolant to the outlet, the outlet being adapted to direct a flow of the shielding coolant around at least a part of the coolant flow. The presence of the additional shielding coolant helps reduce the effect of the surroundings on both the stability and temperature of the main coolant flow.
The shielding coolant flow is preferably provided coaxially with and radially outwardly from the coolant flow as this is the most effective method of shielding the coolant flow from the surrounding environment.
Typically the second supply comprises a coolant store coupled to the isolation line thereby allowing coolant from the isolation line to be used as the shielding coolant. Thus, this advantageously reuses the coolant flowing back along the isolation line so that it can be used to provide the shielding coolant thereby helping to further reduce the amount of coolant required to operate the cryostat. The coolant store operates to store coolant temporarily prior to transfer to the outlet to provide the shielding flow, although this is not essential to the present invention.
Typically the shielding coolant has a higher temperature than the coolant as this also helps prevent the formation of ice on the sample.
The cryostat usually further comprises a gas supply coupled to the outlet, the outlet being adapted to generate a flow of gas and at least part of the coolant flow. This helps further protect both the shielding coolant flow and the coolant flow from the effects of the surrounding environment. Again, the gas flow is preferably arranged coaxially with and radially outwardly from both the shielding coolant flow and the coolant flow.
The isolation line is usually coupled to the supply via a pump, the pump being used to maintain pressure in the supply. This allows the pressure in the supply to be maintained by recirculating coolant thereby helping improve the efficiency of the system.
The supply usually comprises a dewar vessel for storing the coolant although any suitable store can be used.
The coolant is usually liquid helium as this is ideally suited for cooling the sample to the desired temperatures for carrying out X-ray diffraction, neutron diffraction or other similar procedures. However, the system can be used with any suitable cryogen, such as liquid nitrogen, liquid hydrogen, or the like, depending on the circumstances in which it is used.